Even T. Rex Started Small

The skull of Raptorex is dwarfed by that of "Sue," the famous adult Tyrannosaurus rex on display at the Field Museum in Chicago.
Paul Sereno
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Paul Sereno

The skull of Raptorex is dwarfed by that of "Sue," the famous adult Tyrannosaurus rex on display at the Field Museum in Chicago.

Paul Sereno

There's a new discovery that suggests that Tyrannosaurus rex wasn't the first animal that developed the kind of body type associated with gigantic predators.

The fossil, called Raptorex kriegsteini, was found in China and dates back 125 million years — well before T. rex.

The odd thing about Raptorex is that it looks just like T. rex, except it's about 1/90th the size. About 9 feet tall, it weighed more or less what an average human does. It had T. rex's big head, deep jaws, tiny arms, long legs and lanky feet. But Paul Sereno, a paleontologist at the University of Chicago, says it was downsized.

"It is unexpected," says Sereno. "I mean, we really thought that many of these features evolved with large body size, in the course of gaining large body size, as if there were a trend."

But the new fossil suggests that this assembly of predatory body parts wasn't about size.

The 125-million-year-old Raptorex is about 1/90th the size of its descendant Tyrannosaurus rex but was likely one of the most dominant predatory dinosaurs of its time.
Todd Marshall
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Todd Marshall

The 125-million-year-old Raptorex is about 1/90th the size of its descendant Tyrannosaurus rex but was likely one of the most dominant predatory dinosaurs of its time.

Todd Marshall

"What we can now say is that this is a body blueprint for a predator — jaws on legs as it were — that is one of the most successful of the Mesozoic," Sereno says.

That's the era that lasted from 250 million to 65 million years ago. Sereno says Raptorex may have "invented" this body plan, so to speak. Then T. rex came along and supersized it.

'A Great White Shark With Legs'

Sereno notes that the small forelimbs of T. rex are thus not some byproduct of being really big. The arms on Raptorex are also small, just a few inches long.

He thinks arms just weren't that important for any animal whose body was essentially a life support system for its jaws.

"It's sort of like a great white shark with legs," says T. rex expert Greg Erickson at Florida State University.

"What surprises me is that the complex of these very small limbs and a lot of the skull reinforcement and things like that that we see in animals like T. rex were already present in this ancestor," he says. "And it kind of forces us to rethink the significance of the anatomy of animals like T. rex."

The almost-complete skeleton was bought by a private collector and turned over the Sereno, who describes the find in the journal Science. The skeleton is now back in China.